In the Fischer-Tropsch reaction a gaseous mixture of carbon monoxide and hydrogen (synthesis gas) is reacted in the presence of a heterogeneous catalyst to give a hydrocarbon mixture having a relatively broad molecular weight distribution. This product is predominantly straight chain, saturated hydrocarbons which typically have a chain length of more than 5 carbon atoms. The reaction is highly exothermic and therefore heat removal is one of the primary constraints of all Fischer-Tropsch processes. This has directed commercial processes away from fixed bed operation to slurry systems. Such slurry systems employ a suspension of catalyst particles in a liquid medium thereby allowing both the gross temperature control and the local temperature control (in the vicinity of individual catalyst particles) to be significantly improved compared with fixed bed operation.
Fischer-Tropsch processes are known which employ slurry bubble columns in which the catalyst is primarily distributed and suspended in the slurry by the energy imparted from the synthesis gas rising from the gas distribution means at the bottom of the slurry bubble column as described in, for example, U.S. Pat. No. 5,252,613.
The catalyst may also be used in suspension in a reactor comprising at least one high shear mixing zone and a reactor vessel such as the reactor system described in WO 0138269 which is herein incorporated by reference.
However these type of reactors suffer from the disadvantage that separation of the catalyst from the product needs to be employed on a continuous basis and this can be problematic.
Filtration systems are known which utilize either an internal filter in the main body of the reactor or an external filter in a recycle line or a product withdrawal line.
Other filtration systems include a reactor as described in WO 97/12118 with a section of its wall acting as a filter and relies on the turbulent nature of the fluids passing over it to prevent clogging.
Furthermore settling tanks which rely on gravity separation of the catalyst may also be used. However such settling tanks are undesirable for commercial operation because of the very large scale and number required.
Thus for commercial operation of a Fischer-Tropsch process it is essential to provide a continuous and reliable separation system for removing the catalyst from the liquid products such that the catalyst particles can be continuously returned to the reactor without deactivation.